A reversible bimolecular model and in vitro tests were used to determine equilibrium constants and maximal target-to-blood ratios for new derivatives. Theoretical calculations showed that derivatives binding to the beta adrenoceptor or the cholinergic receptor are capable of achieving satisfactory target-to-blood ratios. The most promising parent structure is that of quinuclidinyl benzilate (QNB) with an apparent affinity constant of 3.3 x 10 to the 9th power/M. None of the six iodinated beta adrenoceptor blockers that were synthesized had sufficient affinity to result in high heart-to-blood ratios. Derivatives of QNB and the N-Methyl derivative are candidates for radiolabeling with I-123 or Br-77.